| With the rapid growth of small and medium-sized car ownership in China,the demand for underground garages is increasing.However,most underground garages are closed or semi-closed.The air circulation effect is poor,and the phenomenon of air quality not up to the standard easily appears,which affects the physical and mental health of the people in the garage.Therefore,it is of great significance to study the distribution characteristics of carbon monoxide(CO)and its ventilation system in the underground garage of office buildings.Using field measurement method,this paper studies the frequency of vehicle entry and exit and the garage utilization rate on working and rest days in an office building's underground garage in Guangzhou,the law of CO diffusion in the process of car start-up,the law of hourly variation of CO concentration,and the law of horizontal and vertical distribution,and analyses the factors affecting CO distribution.The local diffusion characteristics of CO in automobile exhaust and the diffusion law of CO under the action of louver and induced fan are studied by numerical simulation method.Finally,taking the most disadvantageous situation of CO concentration in underground garage as the research object,four kinds of mechanical ventilation systems(mechanical exhaust system,mechanical exhaust+ induced draft fan system,mechanical supply and exhaust system,mechanical supply and exhaust + induced draft fan system)are numerically studied to analyze the air quality and air distribution of underground garage,and the influence of ventilation times and vehicle entry-exit frequency on CO concentration in underground garage is analyzed,as well as the removal effect of four ventilation modes on CO concentration is studied.The CO concentration in underground garage increases with the increase of the frequency of car entrance and exit.The average CO concentration in the garage varies from1.5ppm to 12.0ppm in the range of 0.087 ~ 0.496 in the working day.The corresponding frequency ranges of early peak period(7:00 ~ 10:00),noon peak period(12:00 ~ 14:00),late peak period(16:00 ~ 18:00)and peace peak period are 0.220 ~ 0.496,0.181 ~ 0.197,0.331 ~0.417 and 0.087 ~ 0.197,respectively.The utilization rate of garage throughout the day showed a trend of "first increasing and then decreasing",with a peak value of 0.543 from11:00 to 12:00,and the corresponding CO concentration of about 4.5 ppm.In addition,it was observed that the CO concentration in the garage was stratified by "higher upper part and lower under".The concentration of CO in the exhaust gas reaches the maximum value of1934 ppm in the 5th second of start-up,and then decreases gradually.After about 12 s,the concentration is basically stable.When the wind speed in the garage is very low,the CO concentration distribution from the exhaust outlet is gradually rising,and it diffuses along the way.Due to the influence of exhaust velocity,CO moves horizontally after leaving the exhaust port.However,because of the air resistance and heat exchange process,the velocity and temperature of CO decrease rapidly,and the influence of density difference becomes dominant,which makes the diffusion rate of CO in the vertical direction stronger than that in the horizontal direction.The ventilation system and tuyere form of underground garage have important influence on CO concentration distribution.Among all the air outlet forms(side supply side return,down supply down return,down supply side return and side supply down return),the outlet form effect on CO removal efficiency is greater than the return air,and the ventilation effect is the best when the side air supply and side air return are adopted.When the number of ventilation increases from 3 to 7 times per hour,the maximum CO concentration of each system scheme decreases as follows: mechanical exhaust + induced draft fan system is the highest(54.1% ~ 82.8%),the range of mechanical supply and exhaust system is the largest(15.8% ~ 58.6%),and mechanical supply and exhaust + induced draft fan system is the smallest(11.9% ~ 20.4%).Compared with the whole exhaust system,the CO concentration in the scheme with air supply system decreases more greatly.When the frequency of entry and exit increases from 0.4 to 0.8,the CO concentration of the mechanical exhaust system increases the most(103.2% ~ 220.8%)and the mechanical exhaust system increases the least(106.7% ~ 122.0%).Among them,the CO concentration of the scheme with the air supply system increases more greatly.When other conditions are the same,the influence of the frequency of car entry and exit on CO concentration is far greater than that of the ventilation times.That is to say,reducing vehicle exhaust emissions can improve indoor air quality to a greater extent than increasing garage ventilation.According to the air distribution characteristics of each system,the mechanical exhaust system is suitable for garages with regular layout,large opening area and uniform CO distribution.Mechanical exhaust +induced draft fan system has better ventilation effect for garage with larger opening area.Mechanical air supply and exhaust system is suitable for garages with uniform CO distribution.Mechanical air supply and exhaust + induced draft fan system is basically applicable to all garages,which can solve the problems of high CO source intensity(0.4 <frequency of entry and exit ? 0.8)and local CO concentration exceeding the standard. |
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